Steric effect in the energy transfer reaction of Ar(3P2)+N2

Steric effect for N2(C,3Piu) formation in the energy transfer reaction of Ar(3P2)+N2 was directly measured by using an oriented Ar(3P2,MJ=2) beam at a collision energy of 0.06 eV. The N2(C,3Piu) chemiluminescence intensity was measured as a function of the magnetic orientation field direction in the collision frame. A significant alignment effect on the energy transfer probability was observed. The relative reactivity for each magnetic substate in the collision frame sigma|MJ'|was determined to be sigma|2|:sigma|1|:sigma(0)=0.50:0.60:1.00. It is suggested that the observed steric effect is primarily due to the favorable configuration of the 3p orbital for the efficient overlap with the 2sigma(u) molecular orbital of N(2).     

Exact quantum dynamics study of the O(+)+H2(v=0,j=0)-->OH(+)+H ion-molecule reaction and comparison with quasiclassical trajectory calculations

The close-coupling hyperspherical (CCH) exact quantum method was used to study the title barrierless reaction up to a collision energy (E(T)) of 0.75 eV, and the results compared with quasiclassical trajectory (QCT) calculations to determine the importance of quantum effects. The CCH integral cross section decreased with E(T) and, although the QCT results were in general quite similar to the CCH ones, they presented a significant deviation from the CCH data within the 0.2-0.6 eV collision energy range, where the QCT method did not correctly describe the reaction probability. A very good accord between both methods was obtained for the OH(+) vibrational distribution, where no inversion of population was found. For the OH(+) rotational distributions, the agreement between the CCH and QCT results was not as good as in the vibrational case, but it was satisfactory in many conditions. The kk(') angular distribution showed a preferential forward character, and the CCH method produced higher forward peaks than the QCT one. All the results were interpreted considering the potential energy surface and plots of a representative sampling of reactive trajectories.     

Velocity dependence of excitation transfer from Ar(43P2) to Kr

The cross section velocity dependence for the excitation transfer process Ar(4³P₂) + Kr → Ar + Kr(5p[3/2]₂) was studied using a crossed molecular beam time-of-flight method. The cross section increases with increasing velocity, with no apparent threshold down to 500 m/s. A curve crossing model is found to agree qualitatively with the results, and explains the state selectivity of the quenching process.     

Steric effect in the energy transfer reaction of N2 + Rg (3P2) (Rg = Kr, Ar)

Steric effect for the formation of N 2 (B, (3)Pi u ) in the energy transfer reaction of Kr ( (3)P 2) + N 2 has been measured using an oriented Kr ( (3)P 2, M J = 2) beam at a collision energy of 0.07 eV. The N 2 (B, (3)Pi u ) emission intensity was measured as a function of the magnetic orientation field direction in the collision frame. A significant atomic alignment effect on the energy transfer probability was observed. This result was compared with that for the formation of N 2 (C, (3)Pi g ) in the Ar ( (3)P 2) + N 2 reaction. Despite the large difference on the energy transfer cross-section, the atomic alignment dependence for Kr ( (3)P 2) + N 2 is found to be analogous to that for Ar ( (3)P 2) + N 2. It is revealed that the configuration of inner 4p (3p) orbital in the collision frame gives an important role for the stereoselectivity on electron transfer process via the curve-crossing mechanism.     

Reaction dynamics simulations of the identity S(N)2 reaction H(2)O + HOOH(2)(+)--> H(2)OOH(+)+ H(2)O. Requirements for reaction and competition with proton transfer

Despite the fact that the transition structure of the gas phase S(N)2 reaction H(2)O + HOOH(2)(+)--> HOOH(2)(+)+ H(2)O is well below the reactants in potential energy, the reaction has not yet been observed by experiment. Variational transition state RRKM theory reveals a strong preference for the competing proton transfer reaction H(2)O + HOOH(2)(+)--> H(3)O(+)+ HOOH due to entropy factors. Born-Oppenheimer reaction dynamics simulations confirm these results. However, by increasing the collision energy to around 7.5 eV the probability for nucleophilic substitution increases relative to proton transfer. These observations are explained by the presence of the key common intermediate HOO(H)[dot dot dot]H-OH(2)(+) which leads to effective proton transfer, but can be avoided with increasing collision energy. However, the S(N)2 probability remains below 0.2 since successful passage through the TS requires optimum initial orientation of the reactants, excitation of the relative translational motion and good phase correlation between the O-O vibration and the motion of the incoming water.     

Energy dependence of rotational and vibrational distributions of N2(C) for the Ar*(3P0,2) + N2(X) excitation transfer reaction

The Ar* + N₂(X) → N₂(C, v′, N′) + Ar excitation transfer reaction has been investigated experimentally in two different atomic beam experiments. The inelastic cross sections Q_{v′ = 0}(E) and Q_{v′ = 1}(E) to the v′ vibrational level have been measured in the energy range 0.06 ⩽ E(eV) ⩽ 6, using a crossed beam machine. Both cross sections show a behaviour typical for a curve crossing mechanism, with maximum values Q₀ = 8.0 Ų and Q₁ = 1.2 Ų at E = 0.16 eV and E = 0.13 eV, respectively. The oscillatory behaviour of the ratio Q₁(E)/Q₀(E), as first observed by Cutshall and Muschlitz, is also present in our data. Within the model of Gislason et al. the results indicate a decreasing bond stretching with increasing energy. As an alternative we discuss the possibility that the oscillation is due to a different energy dependence of the cross sections for the Ar*(³P₀) and Ar*(³P₂) fine structure states in the mixed beam of metastable Ar*. The vibrational and rotational distributions have also been measured at E = 0.065 eV in a small scale atomic beam-scattering cell experiment, which can be considered as an intermediate between a bulk experiment and a crossed beam experiment. The relative vibrational populations are n_v′ = 100, 16.0, 3.03 and 0.31 for v′ = 0 through 3, with rotational “temperatures” of T_rot,v′ = 1960, 1010, 370 and 130 K. Pronounced deviations (“hump”) of the Boltzmann rotational distributions occur at N′ ≈ 27 for v′ = 0, 1 and 2, with a fractional population of 1, 3 and 11%. For v′ = 0 the “hump” is largely obscured by overlap with the v′ = 1 bandhead. These bimodal distributions are in qualitative agreement with the results of Nguyen and Sadeghi for v′ = 0. The results are discussed within the framework of a curve crossing mechanism with the Ar⁺-N⁻₂ diabatic potential as an intermediate. By assuming equal charges on both N atoms the Coulomb potential of the collinear orientation lies lower (0.45 eV at R = 2.5 Å) than the perpendicular orientation, with the consequence of different transfer probabilities for both orientations. Within a classical model or rotational excitation the final N′ values can be calculated for both orientations, resulting in much higher N′ values for the perpendicular orientation. This mechanism supplies a qualitative explanation for the observed bimodal rotational distributions.     

Ion-molecule rate constants and branching ratios for the reaction of N(3) (+)+O(2) from 120 to 1400 K

The kinetics of the reaction of N(3) (+) with O(2) has been studied from 120 to 1400 K using both a selected ion flow tube and high-temperature flowing afterglow. The rate constant decreases from 120 K to approximately 1200 K and then increases slightly up to the maximum temperature studied, 1400 K. The rate constant compares well to most of the previous measurements in the overlapping temperature range. Comparing the results to drift tube data shows that there is not a large difference between increasing the translational energy available for reaction and increasing the internal energy of the reactants over much of the range, i.e., all types of energies drive the reactivity equally. The reaction produces both NO(+) and NO(2) (+), the latter of which is shown to be the higher energy NOO(+) linear isomer. The ratio of NOO(+) to NO(+) decreases from a value of over 2 at 120 K to less than 0.01 at 1400 K because of dissociation of NOO(+) at the higher temperatures. This ratio decreases exponentially with increasing temperature. High-level theoretical calculations have also been performed to compliment the data. Calculations using multi-reference configuration interaction theory at the MRCISD(Q)/cc-pVTZ level of theory show that singlet NOO(+) is linear and is 4.5 eV higher in energy than ONO(+). A barrier of 0.9 eV prevents dissociation into NO(+) and O((1)D); however, a crossing to a triplet surface connects to NO(+) and O((3)P) products. A singlet and a triplet potential energy surface leading to products have been determined using coupled cluster theory at the CCSD(T)/aug-cc-pVQZ level on structures optimized at the Becke3-Lee, Yang, and Parr (B3LYP)/aug-cc-pVTZ level of theory. The experimental results and reaction mechanism are evaluated using these surfaces.     

Quantum wave packet study of N(2D) + H2 reactive scattering

N(²D) + H₂ → NH + H reaction at zero total angular momentum is studied by using a time dependent quantum wave packet method. State‐to‐state and state‐to‐all reactive scattering probabilities for a broad range of energy are calculated. The probabilities show many sharp peaks that ascribed to reactive scattering resonances. The probabilities for J > 0 are estimated by using the J‐shifting method. The integral cross sections and thermal rate constants are then calculated. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Rotationally resolved reactive scattering: imaging detailed Cl+C2H6 reaction dynamics

The hydrogen atom abstraction reaction of Cl (2P3/2) with ethane has been studied using the crossed molecular beam technique with dc slice imaging at collision energies from 3.2 to 10.4 kcal/mol. The products HCl (v,J) (v = 0, J = 0-5) were state-selectively detected using 2+1 resonance enhanced multiphoton ionization. The images were used to obtain the center-of-mass frame product angular distributions and translational energy release distributions. Two general features were found in all probed HCl quantum states at 6.7 kcal/mol collision energy, and these features have distinct translational energy release and angular distributions, as described for HCl (v = 0, J = 2) in a recent preliminary report [Li et al., J. Chem. Phys. 124, 011102 (2006)]. The results for HCl (v = 0, J = 2) at four collision energies were also compared to investigate the energy-dependent dynamics. We discuss the reaction in terms of a variety of models of polyatomic reaction dynamics. The dynamics of this well studied system are more complicated than can be accounted for by a single mechanism, and the results call for further theoretical and experimental investigations.     

Vibrational inelastic and charge transfer processes in H(+)+H2 system: an ab initio study

State-resolved differential cross sections, total and integral cross sections, average vibrational energy transfer, and the relative probabilities are computed for the H(+)+H2 system using the newly obtained ab initio potential energy surfaces at the full CI/cc-pVQZ level of accuracy which allow for both the direct vibrational inelastic and the charge transfer processes. The quantum dynamics is treated within the vibrational close-coupling infinite-order-sudden approximation approach using the two ab initio quasidiabatic potential energy surfaces. The computed collision attributes for both the processes are compared with the available state-to-state scattering experiments at E(c.m.)=20 eV. The results are in overall good agreement with most of the observed scattering features such as rainbow positions, integral cross sections, and relative vibrational energy transfers. A comparison with the earlier theoretical study carried out on the semiempirical surfaces (diatomics in molecules) is also made to illustrate the reliability of the potential energy surfaces used in the present work.     

Experimental study of the asymmetric charge transfer reaction between Ar+ ions and Fe atoms

We investigate the Ar(+)-Fe asymmetric charge transfer (ACT) reaction using a combination of plasma diagnostics methods and a kinetic model of the afterglow plasma, which allow monitoring of the temporal evolution of the densities of different species. The iron vapor is created inside a discharge cell by cathode sputtering; its density is measured by atomic absorption spectroscopy. The rate coefficient of the reaction is evaluated from the emission intensity decay of Fe(+)? lines pumped by the ACT process in the He-Ar-Fe and Ar-Fe afterglow plasmas. The measurements yield a rate coefficient k = 7.6( ± 3.0) × 10(-9) cm(3) s(-1) at T = 300 K.     

Spectral modulation of a charge transfer reaction of 2-methoxy-4-(N,N-dimethylamino)benzaldehyde inside cyclodextrin nanocage

This article reports modulation of intramolecular charge transfer (ICT) reaction of 2-methoxy-4-(N,N-dimethylamino)benzaldehyde (2-MDMABA) encapsulated within the cyclodextrin nanocavities investigated by steady state and time resolved measurements. The ICT emission, absent in bulk water, originates in the presence of α-, β- and γ-CD with the huge enhancement of local emission. From the Benesi–Hildebrand plot, the stoichiometry of the host–guest inclusion complex is found to be 1:1 for β- and γ-CD whereas 1:1 and 1:2 guest to host complexation occur at low and high concentration of α-CD, respectively. The association constants of the inclusion complexes have also been estimated from the Benesi–Hildebrand plot. The greater binding capability of 2-MDMABA with β-CD than that of other two CDs is further supplemented by time resolved study.     

Polarization in elastic scattering: close-coupling studies on ArN2

We present the results of close-coupling calculations of m_j-dependent differential and integral cross sections forj₁ = 2 → j₂ = 2 rotationally elastic ArN₂ collisions. Two potential surfaces were used with differing long-and short-range anisotropies. If the anisotropy is long-ranged the scattering of an isotropic beam results in a significant angle dependent polarization of the elastically scattered products. To a certain extent this reflects a selective loss of m_j-state population due to rotationally inelastic transitions. For quantization along the initial relative velocity vector or perpendicular to the scattering plane, the depolarization of an initially m_j-state selected beam vanishes in the forward direction and is significantly less than the statistical limit at all angles, which indicates a dynamical conservation of the direction of the molecular rotational angular momentum. By contrast, in the helicity frame depolarization is much more pronounced. The oscillatory structure present in the rotationally inelastic differential cross section does not appear to be quenched by the interference between various m → m′ transitions.     

Communication: rovibrationally selected study of the N2+(X; v+=1, N+= 0-8) + Ar charge transfer reaction using the vacuum ultraviolet laser pulsed field ionization-photoion method

By employing an electric field pulsing scheme for vacuum ultraviolet laser pulsed field ionization-photoion (PFI-PI) measurements, we have been able to prepare a rovibrationally selected PFI-PI beam of N(2)(+)(v(+) = 1, N(+)) with not only high intensity and high quantum state purity, but also high kinetic energy resolution, allowing absolute total cross sections [σ(v(+) = 1, N(+))] for the N(2)(+)(X; v(+) = 1, N(+)) + Ar, N(+) = 0-8 charge transfer reaction to be measured at center-of-mass collision energies (E(cm)) down to thermal energies. The σ(v(+) = 1, N(+) = 0-8) values determined at E(cm) = 0.04-10.00 eV are in good agreement with the theoretical predictions based on the Landau-Zener-Stu?ckelberg formulism. Taking into account the experimental uncertainties, the σ(v(+) = 1, N(+)), N(+) = 0-8, measured at E(cm) = 1.56 eV are found to be independent of N(+).     

Nonstatistical behavior of reactive scattering in the (18)O+(32)O(2) isotope exchange reaction

The recombination of oxygen atoms with oxygen molecules to form ozone exhibits several strange chemical characteristics, including unusually large differences in formation rate coefficients when different isotopes of oxygen participate. Purely statistical chemical reaction rate theories cannot describe these isotope effects, suggesting that reaction dynamics must play an important role. We investigated the dynamics of the 18O + 32O2 --> O3(*) --> 16O + 34O2 isotope exchange reaction (which proceeds on the same potential energy surface as ozone formation) using crossed atomic and molecular beams at a collision energy of 7.3 kcal mol(-1), providing the first direct experimental evidence that the dissociation of excited ozone exhibits significant nonstatistical behavior. These results are compared with quantum statistical and quasi-classical trajectory calculations in order to gain insight into the potential energy surface and the dynamics of ozone formation.     

State-resolved reactive scattering by slice imaging: a new view of the Cl+C2H6 reaction

We present state-resolved crossed beam scattering results for the reaction Cl+C2H6-->HCl+C2H5, obtained using direct current slice imaging. The HCl (v=0,J=2) image, recorded at a collision energy of 6.7+/-0.6 kcalmol, shows strongly coupled angular and translational energy distributions revealing features of the reaction not seen in previous studies. The overall distribution is mainly forward scattered with respect to the Cl beam, with a translational energy distribution peaking near the collision energy. However, there is a substantial backscattered contribution that is very different. It shows a sharp peak at 8.0 kcalmol, but extends to much lower energy, implying substantial internal excitation in the ethyl radical coproduct. These results provide new insight into the reaction, and they are considered in terms of alternative models of the dynamics. This work represents the first genuine crossed-beam study in which a product other than the methyl radical was detected with quantum state specificity, showing the promise of the approach generally for high resolution state-resolved reactive scattering.     

Photodissociation of pyrrole-ammonia clusters by velocity map imaging: mechanism for the H-atom transfer reaction

The photodissociation dynamics of pyrrole-ammonia clusters (PyH·(NH(3))(n), n = 2-6) has been studied using a combination of velocity map imaging and non-resonant detection of the NH(4)(NH(3))(n-1) products. The excited state hydrogen-atom transfer mechanism (ESHT) is evidenced through delayed ionization and presents a threshold around 236.6 nm, in agreement with previous reports. A high resolution determination of the kinetic energy distributions (KEDs) of the products reveals slow (～0.15 eV) and structured distributions for all the ammonia cluster masses studied. The low values of the measured kinetic energy rule out the existence of a long-lived intermediate state, as it has been proposed previously. Instead, a direct N-H bond rupture, in the fashion of the photodissociation of bare pyrrole, is proposed. This assumption is supported by a careful analysis of the structure of the measured KEDs in terms of a discrete vibrational activity of the pyrrolyl co-fragment.     

Significant change of alignment effect by dimer formation in the dissociative energy transfer reaction of Ar(3P2)+(N2O)n and (H2O)n

An alignment effect in the dissociative energy transfer reaction of Ar((3)P(2))+(X(2)O)(n)(X=N,H) was directly measured using an oriented Ar((3)P(2),M(J)=2) beam. The chemiluminescence intensity of N(2)(B,(3)Pi(g)) for (N(2)O)(n) and OH(A,(2)Sigma(+)) for (H(2)O)(n) was measured as a function of the magnetic orientation field direction in the collision frame. The relative reaction cross section for each magnetic substate in the collision frame, sigma(M(J) (') ), was determined. In both the reaction systems, it is observed that the dimer formation significantly enhances the alignment effect and decreases the reactivity, especially for sigma|1| and sigma|2|. A significant contribution of rank 4 moment is recognized in the dimer reaction.     

Synthesis of diaryl acetates and oxindoles via a sequential VNS(Ar)-S(N)Ar three-component coupling reaction

[reaction: see text] The VNS and S(N)Ar reactions combine to form an efficient three-component one-pot route to diarylmethanes. The products of selected diaryl acetates provide modular 3-aryloxindole derivatives.